This invention relates to a control device for a direct clutch allowed to operate at proper timing in a vehicular automatic transmission, essentially comprising: a fluid torque converter; an auxiliary transmission having plural stages of gear trains selectively connected to the output of the torque converter, said gear trains each having a different gear-to-gear speed ratio,and a plurality of hydraulically operated frictional engagement means for actuating the plurality of speed change gear trains, respectively; and a hydraulically operated direct clutch capable of directly coupling the pump and turbine vane wheels of the torque converter together.
The direct clutch, when actuated, operates to directly couple the pump vane wheel of the torque converter with its turbine vane wheel, eliminating the slip loss caused between both the vane wheels, thus largely contributing to improvements in smooth running and fuel saving during cruising. Consequently, it is desired to enlarge the range of use of such a direct clutch as much as possible; however, the use of a direct clutch at the speed ratios over two speed stages poses various problems.
The first problem is that, if the speed ratio is selectively switched to another while the direct clutch is in operation, the shock produced when the speed ratio is changed will tend to become greater than what is produced by a conventional clutch; this is inconsistent with the proposition that the direct clutch is designed to offer smooth and comfortable driving.
Accordingly, the first object of the present invention is to provide a control device used to supply a buffer zone where a direct clutch is not allowed to operate in the boundary between at least two speed ratios at which the direct clutch should be employed, in order to prevent a speed change operation from being performed while the direct clutch is in operation.
The second problem refers to a reduction in power efficiency due to the loss of the torque amplifying function of the torque converter, if the direct clutch is actuated. When the operating zone of the direct clutch is provided at the second (2ND) and third (TOP) speed stages for an automatic transmission with three forward speed stages, for instance, the power efficiency is reduced by the difference between the speed ratios immediately after the speed stage has been shifted up to the third one; because the output torque curve of the engine is normally low during its low speed operation, the power efficiency is particularly felt insufficient in a light load operation where the shifting is carried out a little earlier.
The second object of the present invention is therefore to solve the above second problem by setting the buffer zone as having a changeable width, namely, by making the zone wider in a light load operation, whereby the direct clutch is actuated after the speed of revolution of the engine has been recovered to a certain degree.
In addition, reduction in the power efficiency is undesirable when the engine is operating almost in a full throttle state, for example, when the vehicle is about to enter a speedway from a ramp way to join the flow of other running vehicles or when from necessity it has to outstrip another car, which is running ahead, on a speedway. The power efficiency is extremely reduced in particular when the vehicle is travelling with a relatively high speed stage gear train, such as with the TOP gear. Under such special circumstances, it is advisable to release the operation of the direct clutch even at the cost of smoothness and fuel saving.
As a result, the third object of the present invention is to solve the problem of decrease in power efficiency by releasing the operation of the direct clutch whenever a high output is required.
In the case of cruising at a speed near the maximum speed, however, there are same cases where stillness as well as fuel saving may be also required although the transmission of the high output has been achieved. Accordingly, the fourth object of the present invention is to meet the requirement by allowing the direct clutch to continue to operate at a vehicle speed higher than a certain level during travelling with a gear train of the highest speed stage.
In addition, when the engine output is not required on the occasion of adjusting the distance between one vehicle and another ahead by reducing the opening degree of the throttle valve of the engine during cruising or on the occasion of controlling the vehicle speed during travelilng on downhill road, engine braking will be applied more strongly, if the direct clutch is operating. This is not preferable from the standpoint of fuel-saving. In order to prevent engine braking from being applied particularly in a region where the engine runs at a high speed, the direct clutch should be preferably released from its operating condition.
Therefore, the fifth object of the present invention is to prevent increase in fuel consumption by releasing the actuation of the direct clutch in the region where a engine braking is strongly applied, in order to maintain the effect of engine braking at the same level as that resulting from an automatic transmission without a direct clutch.